The powering of a.c. induction motors generally requires the use of a controller to furnish proper starting, stopping and reversing functions to prevent damage and excessive strain on the motor, on the motor shaft loads, and on the power system. The ideal controller would accomplish this task in a number of ways. It would limit the starting torque so that shaft loads cannot be damaged if excessive torque is applied upon starting. It would limit the starting current which would otherwise cause damage to the motor windings and possibly the power supply. And it would provide overload protection to keep the temperature rise during operation within safe limits.
A distinguishing features of the a.c. induction motor is that it is a singly-excited machine, that is, in normal use an energy source is connected to either only the stator or rotor circuit, typically the field or stator winding. Currents flowing in the field winding create a rotating magnetic field determined by the frequency of the power source applied to the field winding and by the predetermined number of magnetic poles in the machine. The frequency of the power source, therefore, defines the synchronous speed of the motor.
In an induction machine, currents are made to flow in the armature or rotor winding by induction, which creates an induced magnetic field distribution in the armature interacting with the source magnetic field distribution in the field winding to produce a net unidirectional torque. A singly-excited induction machine is capable of producing torque at any speed below synchronous speed. The ratio of the difference between the synchronous speed and the rotor speed to the synchronous speed is called the slip and is directly proportional to the motor torque. Associated with the slip is the slip frequency. It is the frequency of the induced current in the rotor of the machine. The slip frequency is directly related to the difference between the source frequency and the speed of rotor rotation.
The speed of an induction motor driving a constant inertia load may be effectively controlled in an open-loop fashion by generating an output frequency in excess of the desired rotor speed. Open-loop frequency control is unsatisfactory where variable-inertia loads are encountered or if unexpected friction loading occurs. Increase in torque causes the slip frequency to increase, which results in increased motor current at the expense of highter converter output power. If the greater torque resulting from the increased slip frequency does not accelerate the load at the necessary rate, excessive current may cause an overload shutdown of the power source.
Various schemes have been proposed to control the slip frequency in order to limit motor torque and current. Phase-locked loop systems are known for induction motor control. Such systems generally employ a voltage control oscillator driven by a tachometer output signal derived from the rotational displacement of the motor shaft utilizing some form of phase detection to establish control over the slip frequency. The prior art controllers have limited capabilities as compared to the control system herein disclosed.